One known method for abating certain diesel engine exhaust constituents is by use of an exhaust after-treatment system that utilizes a Selective Catalytic Reduction (SCR) of nitrogen oxides. In a typical SCR system, urea or a urea-based water solution is mixed with exhaust gas. In some applications, a urea solution is injected directly into an exhaust passage through a specialized injector device. The injected urea solution mixes with exhaust gas and breaks down to provide ammonia (NH3) in the exhaust stream. The ammonia then reacts with nitrogen oxides (NOx) in the exhaust at a catalyst to provide nitrogen gas (N2) and water (H2O).
As can be appreciated, SCR systems require the presence of some form of urea close to the engine system such that the engine can be continuously supplied during operation. Various urea or urea-solution delivery systems are known and used in engine applications. In known urea injection systems, temperature-related challenges may arise that can affect the electronic and mechanical hardware used to inject the urea. For example, given that the urea is typically injected directly into the engine's exhaust system, the proximity of a urea injector device to hot engine exhaust may lead to injector component overheating, both during system operation as well as after heat saturation following a hot engine shut-down. Moreover, for systems using water-based urea solutions, freezing of the urea solution may cause component damage when the engine is not operating. Other issues include corrosion due to the nature of the urea solution on urea injection components and other, surrounding engine and vehicle components.
Alternatives to urea-based systems can include other types of NOx abatement devices such as lean NOx traps (LNT). LNTs, which can also be referred to as NOx adsorbers, typically include catalysts that reduce oxides of nitrogen emissions from a lean burn internal combustion engine. The nitrogen oxides that may become trapped on the catalyst can be released when the engine is operated in a rich burn as nitrogen gas and oxygen or water, in the presence of hydrogen. It has been suggested in the past that hydrogen gas may aid in regenerating a LNT, but no viable system has been implemented to date in production.
Hydrogen generation by fuel reformation is known from a different area of technology, fuel cells. For example, U.S. Pat. No. 7,066,973 (the '973 patent) describes an integrated reformer and shift reactor for producing hydrogen gas for a fuel cell. As is known, the fuel reformers such as the one described in the '973 patent are costly, complicated, and include various structures and operating areas that aim in reducing CO content in the hydrogen gas produced, because CO is detrimental to the operation of fuel cells for which the hydrogen gas is produced.